Electrically programmable fuse (eFUSE) is commonly used in integrated circuits including CMOS integrated circuits for a variety of reasons. For example, eFUSE is used to form permanent connections in an integrated circuit after the integrated circuit is manufactured. eFUSE is also used in circuit repairs, i.e., to interconnect a redundant circuit when the primary circuit fails in order to improve product yield. eFUSE can also be used as permanent memory to store information on chips such as chip ID or critical system boot codes. Additionally, eFUSE can be used to adjust the speed of a circuit by adjusting the resistance of a current path formed by the eFUSE.
FIG. 1A illustrates a top view of a known eFUSE 102. FIG. 1B illustrates a cross sectional view of eFUSE 102 of FIG. 1A. eFUSE 102 has a cathode contact region 106, an anode contact region 108, and a fuse link 104. Fuse link 104 interconnects cathode contact region 106 to anode contact region 108. The fuse link 104 of eFUSE 102 has an underlying poly-silicon layer 112 and an overlying silicide layer 110. eFUSE 102 also has an oxide layer 114 separating poly-silicon layer 112 from nitride layer 116. Alternatively, as illustrated in FIG. 1C, eFUSE 102 may have an underlying oxide layer 122 and overlying silicide layer 118, separated by a silicon layer 120. A nitride layer 124 overlays silicide layer 118.
eFUSE 102 is programmed by electromigration of silicide in fuse link 104 from cathode contact region 106 to anode contact region 108, as follows. A voltage potential is applied across fuse link 104 via anode contact region 108 and cathode contact region 106, such that the resultant current has a magnitude and direction to initiate electromigration of silicide from the cathode contact region side of the semiconductor fuse link 104 and to create a gap in silicide, thereby reducing the conductivity of the fuse link 104.
The programming of an eFUSE is sensitive to process and power supply variations. Process variations change the fuse character, such as overall resistance and sub-components resistances, and thus lead to change of the optimal programming current. Power supplies used for fuse programming may also experience variations, such as power droop, on a given product implementation, i.e. the potential variation at fuse to be programmed due to parasistic wiring resistance and leakage current in the circuits. As a result, a fuse may be programmed with either too little current or too much current which leads to undesirable outcomes. For example, the programming yield may suffer from either end. In addition, the programmed fuse may heal in subsequent manufacturing process such as test and packaging, or worse in the field which results in a failing product in service.
FIG. 2 illustrates an example eFUSE 202 after the eFUSE 202 has been programmed with too much current. eFUSE 202 has experienced damage 206 at cathode contact 204 resulting from over programming that partially migrated the cathode contact material, e.g., the contact liner material and contact itself. As a result, copper metal wire above the contact can be exposed and readily diffuse into the programmed fuse link when the fused part is subjected to elevated temperature, such as those during the subsequent manufacturing process. For example, a packaging process subjects eFUSE 102 to heat as high as 360° C. This renders the eFUSE back to its original pre-programmed state. FIG. 3A illustrates an example of a failed eFUSE 302 resulting from copper diffusing from the cathode contact 304 into fuse link 306. FIG. 3B is an elemental analysis 308 of the failed eFUSE of FIG. 3A showing the concentration of copper 310 in the programmed fuse link 306 that electrically reconnects the cathode and anode contacts indicating a pre-programmed state.
In addition, the lithography used to manufacture the known eFUSE 102 is imprecise. FIG. 4A illustrates eFUSE design 402 having a right angle corner 410. Physically implementing the design, however, results in rounded corners 430 as illustrated by eFUSE 450 in FIG. 4B. Furthermore, corner rounding is highly variable. Several factors contribute to variations in corner rounding including tool focus, dosage, optical proximity correction (OPC), light wavelength, photoresist, etc. Thus, corner rounding contributes to fuse variability in terms of optimal programming condition and programming yield.
An object of the present invention is to improve the safety window for fuse programming through heat conduction engineering so that programming occurs at desired location, the center region of fuse link, away from the susceptible cathode contact. Another object of the present invention is to design an eFUSE to tolerate higher programming current. Another object of the present invention is to minimize the sensitivities of fuse programming to process variations such as corner rounding effect.